1. Field of the Invention
The present invention relates to an optical device fixing device, and particularly to a prism mounting device for an optical device using light beam which passes through a prism or lens, a lens fixing device for aligning and fixing the center of a lens with a prescribed optical axis, and a reflecting mirror fixing device for fixing a reflecting mirror of an optical device.
2. Description of the Related Art
To open a locked door or to allow only registered persons to access to valuable data, a judging device according to fingerprint checking has been developed.
A conventionally known developed individual fingerprint checking device comprises a prism, a lens, and an image processing circuit. FIG. 12 shows such a device as a schematic vertical sectional view.
The device shown in FIG. 12 has a fingerprint checking device housed into a case 1 and a top lid 2 mounted at the top. Within the case 1, a prism 4 and a lens 5 are disposed substantially in a horizontal direction, and an emission unit 3 is positioned below the prism 4. The upper part of the prism 4 is in contact with the top lid 2. The prism 4 is positioned with its center axis aligned with the optical axis of the lens 5. A mirror 7 is positioned to reflect the light coming out of the lens 5, and the light reflected on the mirror 7 enters an image pickup device 8. The prism 4 is fully covered with a light impermeable material excluding a surface opposed to the emission unit 3, a surface on which a finger 9 is positioned, and a surface from which light is entered into the lens 5.
In the device shown in FIG. 12, when light is emitted from the emission unit 3 to the prism 4, the emitted light passes through the prism 4 and also the top lid 2. When the finger 9 is positioned on a lighted area to check a fingerprint, the light from below is substantially absorbed at valley portions of the fingerprint, and the light is mostly reflected on ridge portions of the fingerprint to enter the prism 4.
As indicated by the arrows, the light goes downward within the prism 4, and reflects again on the prisms's surface opposed to the emission unit 3 (a bottom face 4a of the prism 4). In view of the shape of the prism 4, the surface of the prism 4 opposite from the lens 5 forms a reflection surface 10 for the light, so that the light reflected on the bottom face 4a of the prism 4 is reflected on the reflection surface 10, and goes straight through the prism 4 to the lens 5.
The lens 5 is covered by a lens holder 6 and held within the case 1. Accordingly, the position of the lens 5 can be adjusted to focus the light, which has advanced through the prism 4 toward the lens 5, beyond the lens 5 by the lens 5. By adjusting to focus on the mirror 7, the fingerprint image is enlarged by the lens 5, and the light reflected on the mirror 7 reaches the image pickup device 8 to process the fingerprint as a digital image, for example.
To process a small line pattern like a fingerprint as a digital image, the dimensions of the prism 4 and the lens 5 are prescribed, and the dimension of the case 1 for accommodating them is roughly determined accordingly. But, as to the light beam for fingerprint checking, in order to accurately form a fingerprint image on the image pickup device, it is necessary to accurately set the positions of the prism 4 and the lens 5 within the case 1.
Therefore, a position adjusting means has been used as shown as schematic views in FIG. 13 and FIG. 14.
FIG. 13 is a diagrammatic sectional view showing a position adjusting means. In FIG. 13, the prism 4 is in contact with the case 1 by being pushed slightly by springs 11, 12 from two sides toward the end on the lens side in the case 1. Strength and direction of the springs 11, 12 are adjusted so that the optical axis of the prism 4 agrees with the optical axis of the lens 5 at the position where the prism 4 is pushed within the case 1 by the springs 11, 12.
FIG. 14 is similar to FIG. 13 except that screws 13, 14 are used instead of the springs 11, 12 used in FIG. 13. The screws 13, 14 are used in the same way as the springs.
In the prior arts shown in FIG. 13 and FIG. 14, the direction and strength to push the prism 4 in the case 1 toward the lens are adjusted by the springs 11, 12 or the screws 13, 14, so that prescribed alignment of the optical axes of the lens 5 and the prism 4 is easily made.
But, when an external impact is applied to the case 1 after making the adjustment, and even if it is a small impact, the pressure and the position to apply the pressure to the prism 4 by the springs 11, 12 are changed, and the optical axis of the prism 4 is deviated from the optical axis of the lens 5 and when the screws 13, 14 are used, the optical axes are deviated when the screws are loosened.
Meanwhile, various devices have been proposed to fix the lens 5.
FIG. 15 is a diagram for showing a first prior art.
A cylindrical lens guide 15 is integral with the case 1 and provided in a vertical position with respect to the surface of the prism 4. A screw hole 16 is formed on the side face of the lens guide 15, and a fixing screw 17 is driven into the screw hole 16.
In the fixing device configured as described above, the prism 4 is mounted, the lens 5 is inserted into the lens guide 15, and the bottom of the lens 5 is contacted to one surface of the prism 4. In this state, the fixing screw 17 is tightened to fix the lens 5.
FIG. 16 shows a second prior art.
A cylindrical lens guide 18 is integral with the case 1 and provided in a vertical position with respect to the surface of the prism 4. An annular guide claw 19 is formed at the leading end of the lens guide 18. And, a lens holder 20 is provided to cover the side face and bottom of the lens 5.
In the fixing device as described above, the lens 5 is inserted into the lens guide 18 prior to mounting the prism 4. When the prism 4 is mounted in this state, the bottom of the lens 5 is pressed against the surface of the prism 4. By such a pressure, the curved part of the lens 5 is caught and fixed by the guide claw 19. And, the bottom and side face of the lens 5 are fixed in close contact with the lens guide 18 and the prism 4 through the lens holder 20.
FIG. 17 is a diagram showing a third prior art.
A cylindrical lens guide 21 is integral with the case 1 and provided in a vertical position with respect to the surface of the prism 4. The leading end of the lens guide 21 has a conical surface within it and a conical claw 22 which has its center open. And, a lens holder 20 is provided to cover the side face and bottom of the lens 5.
In the fixing device configured as described above, the lens 5 is inserted into the lens guide 21 prior to mounting the prism 4. When the prism 4 is mounted in this state, the bottom of the lens 5 is pressed against the surface of the prism 4. By such a pressure, the curved part of the lens 5 is advanced in sliding contact with the surface of the conical claw 22, and the central axis of the lens 5 is aligned with the central axis of the conical claw 22. And, the bottom and side face of the lens 5 are fixed in close contact with the lens guide 21 and the prism 4 through the lens holder 20.
But, the first and second prior arts have to form the lens guides 15, 18 having accurate dimensions to align the center axis of the lens with the optical axis of the prism 4. Particularly, the dimensions are varied in a lens polishing process or other processes. It is very hard to form the lens guides 15, 18 having the dimensions conforming with such variations.
And, in the first prior art, when the fixing screw 17 was being tightened, the lens 5 was often inclined or displaced and fixed as shown in FIG. 18.
On the other hand, in the second prior art, a play is produced between the lens guide 18 and the lens holder 20 due to aging or the like. The center line (the dashed in the drawing) of the lens 5 was often deviated from the optical axis (the dotted line in the drawing) of the prism 4 because of such a play as shown in FIG. 19.
And, in the third prior art, a partial pressure which is vertically applied to the face where the curved part of the lens 5 is in contact with the conical surface is small, so that a frictional force applied between them is lowered. Since the curved part of the lens 5 is fixed by such a small frictional force, the lens 5 is easily displaced when an external shocking vibration is applied.
On the other hand, the reflecting mirror in an optical device, which is used to change the direction of the light beam by about 90 degrees, mostly uses a glass mirror.
Lately, a synthetic resin plate is often used instead of glass and metal-plated to produce the reflecting mirror in order to reduce the overall weight of the device used. Such a reflecting mirror has its back pushed by a metallic spring to stably mount the reflecting mirror.
FIG. 20 is a perspective view showing the positional relation between the reflecting mirror and the plate spring in a prior art. In FIG. 20, a reflecting mirror 23 has its back face supported by a plate spring 24. A screw hole 25 for fixing the plate spring is formed at the end of the plate spring 24, and the plate spring 24 is fixed to a case not shown.
FIG. 21 is a side sectional view of FIG. 20. In FIG. 21, the light beam L1 entered from the left in the drawing is reflected on the reflecting mirror 23 and directed downward as shown by L2.
In FIG. 21, when the original adjustment has been completed, the reflecting mirror 23 and the plate spring 24 are desired to keep a contacted state at a tangent in the horizontal direction on the back face of the reflecting mirror 23 through a point Q. But, when the leaf spring 24 is fixed by the screw, the reflecting mirror 23 and the plate spring 24 are fixed at one point (contact point) only on some midpoint of the tangent. When the material to form the reflecting mirror 23 is rigid and heavy like metal, light is not reflected in a direction against expectation even when the contact point is in a fixed state, but when the material is weaker and lighter than the reflecting mirror 23, light is reflected in a direction against expectation, so that the device is required to be adjusted again.